1. Field of the Invention
The present invention is related to fluid dispensing systems and methods associated with their use. More particularly, the present invention relates to those applications of a wide range of fluids with different viscosities where preciseness and accuracy of the amount of fluid dispensed is important.
2. Background Information
Dispensing consistent, controllable, and accurate amounts of fluids of varying viscosities remains a long standing issue for manufacturers and others where an efficient, accurate and precise amount of liquid dispensed supports a high quality product.
Herein “accurate” refers to how exact is an amount dispensed, and “precise” refers to how repeatable is the amount dispensed.
Herein the term “dot” or “bead” refers to the quantity of fluid dispensed, and “fluid” refers to liquids or suspensions that react as do liquids.
In a product manufacturing environment it is important to accurately control the amount of fluid being dispensed. If, for example, too little adhesive is dispensed the assembly may not have the structural integrity of a quality product; too great a dot or bead and the excess may ooze and make unwanted contact with other areas and/or present an unsightly finished product. In addition, low viscosity fluids may leak after the dot or bead is dispensed. To control such leakage, the controller may incorporate a backoff function. In the example of a controller moving a syringe piston forward a set distance to dispense a dot or bead, the controller will then reverse or “backoff” the piston motion creating a partial vacuum to prevent the fluid from leaking.
Known fluid dispensers may include a syringe with air pressure (pneumatic) driving the syringe piston. These systems often leak and lose both air and vacuum (negative) pressure resulting in imprecise and inaccurate dots and beads.
In pneumatic dispensers backoff is accomplished by creating a vacuum behind the piston. Positive air pressure behind the piston drives the piston forward dispensing the fluid, then a vacuum, or negative air pressure, replaces the positive air pressure, thus stopping and reversing the piston motion to pull the fluid back from the needle opening, preventing leakage. Multiple valves and solenoids are used in such systems.
In mechanical systems, backoff occurs by reversing the motor a predetermined distance. A predetermined backoff distance, however, does not account for the reduction of the fluid and the increase of the air space behind the piston in the syringe, as the fluid is dispensed. It has been found that inaccurate, imprecise amounts of fluid are delivered as the syringe fluid is depleted since the predetermined distance results in a different pressure change as the air space behind the piston changes.